JP4210191B2 - Method for producing austenitic stainless steel sheet with excellent surface uniformity - Google Patents

Description

  The present invention relates to a method for producing a steel sheet by annealing and pickling a hot-rolled steel sheet of austenitic stainless steel.

As shown in Non-Patent Document 1, the method of hot rolling austenitic stainless steel continuous cast slab, solution treatment, pickling and manufacturing a steel plate is as follows. Has been done to manufacture. In addition, for example, in Patent Document 1, as a method of manufacturing a hot-finished steel plate or a thick plate product, an oscillating mark is used when manufacturing a continuous cast slab without performing surface maintenance. In order to prevent the portion from remaining on the steel plate after pickling, it is disclosed as a method for adjusting the heating atmosphere during solution treatment and the shot blasting conditions before pickling.
"Stainless Steel Handbook" 3rd edition (issued January 24, 1995), page 745 JP 2000-160246 A

  However, even when manufactured by such a conventionally disclosed method, it was found that a pattern having a polka dot shape, a flowing water shape or a map shape is formed on the surface of the steel plate after pickling, resulting in a steel plate with poor surface uniformity. . The object of the present invention is to fundamentally prevent a pattern having a polka dot shape, a flowing water shape or a map shape generated on the surface of a steel plate after pickling, which could not be achieved by the prior art, and uniform surface quality after pickling. It is to provide a good steel sheet excellent in the above.

In order to achieve the above object, the present invention comprises the following arrangement.
(1) Austenitic stainless steel is cast by a continuous casting apparatus, hot rolled, annealed, and then subjected to descaling pickling. In the steel sheet manufacturing method, cooling after hot rolling is performed at a steel plate temperature of less than 600 ° C. A method for producing an austenitic stainless steel sheet, wherein the process is allowed to cool to room temperature and then water-cooled .
(2) In a method for manufacturing a steel sheet in which austenitic stainless steel is cast by a continuous casting apparatus, hot-rolled, annealed, and then descaled and pickled, cooling after hot rolling is performed at a steel plate temperature of 600 ° C. or higher and 675 ° C. A method for producing an austenitic stainless steel sheet, characterized in that water cooling is started from a temperature of ℃ or less and the annealing condition is maintained at a temperature of 1070 ℃ or more and less than 1100 ℃ for 5 minutes or more .

  The present invention fundamentally prevents the pattern having a polka dot shape, flowing water shape or map shape, which is difficult to achieve with the prior art by the present invention, which occurs on a product pickling plate of austenitic stainless steel, and uniform surface quality after pickling. It is possible to make a good steel sheet with excellent properties.

  As a result of various detailed studies, the present inventors have fundamentally prevented a pattern having a polka dot shape, a flowing water shape or a map shape generated on the surface of a steel strip after pickling of an austenitic stainless steel plate, and pickling. As a method of making a good steel sheet with excellent surface quality afterwards, pay attention first to the cooling conditions after hot rolling, and know that the pattern can be prevented by adjusting the temperature to start water cooling from standing to cool did. Hereinafter, it demonstrates according to an experimental result.

  For SUS304 steel, a continuously cast 200 mm slab was hot rolled to a thickness of 5 mm, and the steel strip after hot rolling was water cooled between 400-900 ° C., annealed at 1050 ° C. for 10 minutes, then water cooled, and shot blasted. Mechanical descaling and pickling with nitric hydrofluoric acid solution.

  The source of the polka dot, flowing water or map pattern generated after pickling is at the time of water cooling immediately after the heat treatment, and at this point the peeled portion of the oxide scale becomes irregular irregularities of 100 to 500 mm in size. The pattern had a polka dot shape, a flowing water shape or a map shape without being repaired by subsequent shot blasting or pickling.

As a result of investigating the relationship with the cooling conditions after hot rolling with respect to the occurrence of the pattern, (1) there was no occurrence in the material that was allowed to cool after hot rolling. (2) As shown in Table 2 , the result of water cooling in the middle of cooling after hot rolling was characterized in that a pattern was generated when the water cooling start temperature was 600 to 800 ° C. (3) When the steel sheet capable of water cooling at 600 to 800 ° C. was further allowed to cool and then cooled to below 600 ° C., no pattern was generated. (4) Even when the steel plate that can be cooled with water at 600 to 800 ° C is reheated or heated up to a temperature exceeding 800 ° C, the temperature at which the water cooling is actually started exceeds 800 ° C. There was no outbreak.

  Thus, when water-cooling is performed in the middle of cooling after hot rolling, the reason for generating a pattern when the water-cooling start temperature is 600 to 800 ° C. is not clear, but the grain boundary in the temperature range of 600 to 800 ° C. is not clear. It is presumed that Cr deficiency occurs due to the formation of chromium carbide, sensitization is likely to occur, and sensitization macroscopic heterogeneity occurs due to subtle differences in oxide scale generated on the surface after hot rolling. . Therefore, by avoiding the start of water cooling in the temperature range of 600 to 800 ° C., macroscopic nonuniformity of sensitization can be eliminated, and the pattern of the product plate can be eliminated.

  Next, the inventors pay attention to the annealing conditions after hot rolling, and prevent the pattern by making the annealing temperature and time appropriate even when the water cooling start temperature after hot rolling is 600 to 800 ° C. I found out that I can do it. Hereinafter, it demonstrates according to an experimental result.

  For SUS304 steel, a continuously cast 200 mm thick slab was hot rolled to a thickness of 5 mm, and the steel sheet after hot rolling was subjected to an annealing temperature with respect to a steel sheet that was water-cooled from 700 ° C. after hot rolling under the condition that a pattern was generated in the experiment. Were processed at various temperatures between 1000 ° C. and 1200 ° C. and the time between 0 and 40 minutes, mechanically descaled with shot blasting, and pickled with nitric hydrofluoric acid solution. The results of investigating the relationship between annealing conditions and patterns are summarized in FIG.

From FIG. 1, the pattern becomes mild if held at a temperature of 1070 ° C. or higher for 5 minutes or more as an annealing condition, and more desirably, if the holding is performed at a temperature of 1100 ° C. or higher for 15 minutes or more, there is no pattern, after pickling A good steel sheet with excellent surface quality and uniformity can be obtained.
In this way, the reason why the pattern is reduced or eliminated by securing the annealing temperature and time is not clear, but the macro-uniformity of the sensitization is eliminated by annealing at a high temperature for a long time. It is estimated that.

Next, the reason for limitation will be described.
The reason for limiting to austenitic stainless steel was limited to the phenomenon that the present invention has a problem due to a phenomenon peculiar to austenitic stainless steel containing 7 to 22% Cr and 4 to 12% Ni. . Further, for the same reason, the manufacturing process is limited to a steel plate which is cast by a continuous casting apparatus, followed by water cooling immediately after hot rolling, annealing, and then descaling pickling.

As described above, the cooling condition after the hot rolling is that the water cooling is not started when the steel plate temperature is 600 to 800 ° C., so (1) the steel plate temperature is less than 600 ° C. (2) Water cooling is started from a temperature where the steel plate temperature exceeds 800 ° C., (3) Reheating or heating up to a temperature exceeding 800 ° C. It is necessary to satisfy any one of conditions for starting water cooling after allowing the water to cool.
In (1), after the steel sheet temperature is less than 600 ° C., it may be cooled with water or may be allowed to cool. However, from the viewpoint of productivity, it is preferable to cool with water.

  Further, even when water cooling is started at a steel plate temperature of 600 to 800 ° C. after the hot rolling, as a subsequent annealing condition, it is held at a temperature of 1070 ° C. or more for 5 minutes or more, more preferably at a temperature of 1100 ° C. or more. This is because by holding for 15 minutes or more, a pattern is not generated on the surface of the steel plate after pickling, and a steel plate having excellent uniformity can be obtained. Of course, this condition may be carried out when the water cooling start temperature after hot rolling is less than 600 or more than 800 ° C.

Austenitic stainless steel SUS304, SUS304L, SUS316L steel containing the components shown in Table 1 and the balance Fe and inevitable impurities was hot-rolled to a thickness of 2 to 6 mm from a 150 to 250 mm thick slab cast by a continuous casting apparatus. . The steel sheet after hot rolling was subjected to water cooling between 400 to 900 ° C. shown in Table 2 and allowed to cool without water cooling.
After changing the annealing temperature and time, annealing was performed, followed by water cooling, mechanical descaling by shot blasting, and pickling with a nitric hydrofluoric acid solution containing 150 g / L nitric acid-70 g / L hydrofluoric acid at 40 to 55 ° C.

When the hot-rolled steel sheet is allowed to cool without water cooling, or even if it is water-cooled between 600-800 ° C., if it is kept at a temperature of 1070 ° C. or more for 5 minutes or more as an annealing condition, a temperature of 1100 ° C. or more is further increased. When it was held for 15 minutes or more, it became good. On the other hand, when it was water-cooled between 600-800 degreeC and the annealing conditions were inadequate, the pattern which had a polka dot shape, flowing water shape, or map shape was produced after the final pickling.
Furthermore, based on the experimental results shown in Table 2 below, the cooling range after hot rolling in the present invention is such that the steel sheet temperature starts water cooling from a temperature of 600 to 675 ° C, and the annealing condition is a temperature of 1070 ° C or higher and lower than 1100 ° C. For 5 minutes or longer.

The figure which shows the relationship between the presence or absence of the pattern which appears after a descaling pickling with respect to the annealing temperature and time of austenitic stainless steel.

Claims (2)

In the steel sheet manufacturing method in which austenitic stainless steel is cast by a continuous casting machine, hot rolled, annealed, and then descale pickled, cooling after hot rolling is released until the steel sheet temperature is less than 600 ° C. method of manufacturing an austenitic stainless steel sheet, characterized in that water cooling after cooling. In the manufacturing method of a steel sheet in which austenitic stainless steel is cast by a continuous casting apparatus, hot-rolled, annealed, and then descale pickled, cooling after hot rolling is performed with a steel sheet temperature of 600 ° C. or higher and 675 ° C. or lower. A method for producing an austenitic stainless steel sheet, wherein water cooling is started from a temperature , and the annealing condition is maintained at a temperature of 1070 ° C. or higher and lower than 1100 ° C. for 5 minutes or longer .

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